WASHINGTON (AP) — Despite criticism of Fannie Mae by Republican presidential candidate Mitt Romney, his campaign accepted nearly $280,000 in donations raised by a registered lobbyist who once represented the government mortgage giant and whose clients now include a private equity firm and the drug company Pfizer.

Yet Romney has not identified all of his so-called fundraising "bundlers" who have raised hundreds of thousands of dollars, even after President Barack Obama's re-election campaign released the names of his top fundraisers. Rick Santorum and Newt Gingrich also haven't disclosed their bundlers. Ron Paul's campaign has said it doesn't use them. For more than a decade, since the election of George W. Bush in 2000, presidential campaigns have identified their bundlers.

In an age of "super" political action committees, which can pull in millions of dollars from anonymous donors, bundlers still matter to modern presidential campaigns. Like Wayne Berman, the chairman of Ogilvy Government Relations and former Fannie Mae lobbyist who gathered $279,075 for Romney in 2011, these are well-connected executives who collect or direct multiple individual contributions of up to $2,500 to a campaign in amounts that can range from $50,000 to more than $500,000.

The lack of disclosure prevents voters from knowing who wields influence within a presidential campaign. Keeping their identities secret could end up stinging Romney — like the mishandled release of his income tax returns — if voters conclude he is withholding politically damaging information.

Federal law requires only that candidates disclose the identities of bundlers who also are registered lobbyists, which the Romney campaign has done. Berman and 15 other lobbyists representing a wide range of interests raised nearly $2.2 million for Romney in last year, according to Federal Election Commission records. Their clients included investment firms and a mortgage processing company accused of "robo-signing" foreclosure documents.

But just as disclosing income tax statements is commonplace for leading presidential candidates, voluntarily identifying bundlers has also become standard practice.

A corporate executive or a Wall Street broker who acts as a bundler can wield just as much influence as a lobbyist. "Bundlers pose a very similar threat of corruption of candidates and office holders just as unlimited contributions would if made directly to the candidate or office holder," said Paul Ryan, associate legal counsel at the nonprofit Campaign Legal Center.

Ryan said the rise of super PACs, which can't lawfully coordinate their spending with a candidate's campaign, haven't diluted the importance of bundlers. "At the end of the day, candidates want money in their own campaign accounts because they have full control over how it is spent," he said.

Andrea Saul, a Romney spokeswoman, said the campaign discloses all of the information about its donors that is required by law. "Anyone who is interested can review it publicly," she said. Saul's response suggested that Romney's campaign would continue to withhold the identities of its bundlers who are not registered lobbyists.

Obama's re-election campaign on Jan. 31 released the names of more than 440 bundlers who collected close to $75 million in 2011, including 61 people who each raised at least $500,000. The Obama campaign said it does not accept contributions from lobbyists. But the list includes people involved in the business of influencing government.

Michael Kempner, for example, is president and chief executive officer of MWW Group, a public relations firm with a large lobbying business. Kempner, who raised more than $500,000 for Obama's reelection bid, is not himself a registered lobbyist.

Also on Obama's list are two fundraisers linked to Solyndra LLC, the California solar company that received a $528 million Energy Department loan and then later went bankrupt, prompting a federal investigation. Obama's re-election campaign also received about $200,000 in contributions bundled by family members of a Mexican casino owner who fled the U.S. after facing drug and fraud charges. The Obama campaign said it has returned the money.

More than 1,000 bundlers collectively raised $152.2 million for the 2008 campaigns of Obama and his opponent, Republican Sen. John McCain, according to the nonprofit Center for Responsive Politics.

Romney's Republican opponents also haven't identified their bundlers. But the stakes are higher for Romney, who holds a huge fundraising lead over his rivals. Romney's campaign raised nearly $57 million in 2011 from individual contributions, which are capped by law at $2,500 each for primary and general elections.

Santorum, the former Pennsylvania senator who stunned Romney by sweeping contests last week in Minnesota, Missouri and Colorado, raised just $2.2 million last year. Gingrich, the former House speaker, raised $12.6 million last year and Paul, a Texas congressman, collected $25.5 million.

Santorum, Gingrich and Paul have not reported having any registered lobbyists who are bundlers, according to the FEC records.

Money is the lubricant for presidential campaigns. But Santorum, seeking to maintain momentum from his recent victories, sought to use Romney's financial dominance against him with a David-versus-Goliath message.

"We're not going to win this race the way Gov. Romney has won the states he's won already, by outspending his opponent by 5-to-1 and beating them up. He's not going to outspend Barack Obama 5-to-1," Santorum said last week in Dallas. "How are you going to win an election if your greatest attribute is 'I'll spend more money than the other person?'"

Another lobbyist-bundler, T. Martin Fiorentino Jr. of The Fiorentino Group, represents Lender Processing Services, a mortgage processing company in Jacksonville, Fla. Nevada's attorney general filed a civil lawsuit against LPS in December that accuses the company of filing fraudulent documents in the months before the state's housing market collapsed. LPS has said the allegations are false and last month asked a judge to dismiss the case.

Romney may be keeping the names of his bundlers under wraps to avoid drawing attention to the emphasis his campaign is placing on large individual contributions and the role that Wall Street and the financial services industry are playing in helping to generate money, said Anthony Corrado, a campaign finance expert and a professor of government at Colby College in Maine.

"Essentially $38 million of the $57 million he raised came from people who gave $2,500," Corrado said. "That, to me, suggests a significant amount of bundling activity because those are the people capable of soliciting the larger checks."

Disclosing bundlers can have advantages. Bush embraced bundlers during his presidential campaigns, publicizing their names and rewarding them with titles such as Ranger and Pioneer. That served a dual purpose. The campaign could claim it was being transparent about its fundraising while at the same time motivating others to join an elite club.

"By making all of this public, the candidates can insulate themselves from the charge that they have something to hide," Corrado said, "and at the same time create a culture of fundraising amongst these individuals that tends to spur them to want to do more."